1. Field of the Invention
The invention generally relates to the field of microlithography, and in particular to projection exposure apparatus or mask inspection apparatus. The invention is particularly concerned with correcting, or more generally changing, optical wavefronts in such apparatus.
2. Description of Related Art
Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other micro-structured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultra-violet (DUV), vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection lens. A typical value for the magnification is β=±¼.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning means. Commonly used masks contain opaque, transparent or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example.
One of the essential aims in the development of projection exposure apparatus is to be able to lithographically produce structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus. Furthermore, the more devices can be produced on a single wafer, the higher is the throughput of the production process.
The size of the structures that can be generated depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm or 157 nm and thus lie in the deep or vacuum ultraviolet spectral range. Also apparatus using EUV light having a wavelength of about 13 nm are meanwhile commercially available. Future apparatus will probably use EUV light having a wavelength as low as 6.9 nm.
The correction of aberrations (i.e. image errors) is becoming increasingly important for projection objectives with very high resolution. Different types of aberrations usually require different correction measures.
The correction of rotationally symmetric aberrations is comparatively straightforward. An aberration is referred to as being rotationally symmetric if the wavefront deformation in the exit pupil of the projection objective is rotationally symmetric. The term wavefront deformation denotes the deviation of an optical wave from the ideal aberration-free wave. Rotationally symmetric aberrations can be corrected, for example, at least partially by moving individual optical elements along the optical axis.
Correction of aberrations that are not rotationally symmetric is more difficult. Such aberrations occur, for example, because lenses and other optical elements heat up rotationally asymmetrically. One aberration of this type is astigmatism.
A major cause for rotationally asymmetric aberrations is a rotationally asymmetric, in particular slit-shaped, illumination of the mask, as it is typically encountered in projection exposure apparatus of the scanner type. The slit-shaped illuminated field causes a non-uniform heating of those optical elements that are arranged in the vicinity of field planes. This heating results in deformations of the optical elements and, in the case of lenses and other elements of the refractive type, in changes of their refractive index. If the materials of refractive optical elements are repeatedly exposed to the high energetic projection light, also permanent material changes are observed. For example, a compaction of the materials exposed to the projection light may occur, and this compaction results in permanent local changes of the refractive index. In the case of mirrors the reflective multi-layer coatings may be damaged by the high local light intensities so that the reflectance is locally altered.
The heat induced deformations, index changes and coating damages alter the optical properties of the optical elements and thus cause aberrations. Heat induced aberrations sometimes have a twofold symmetry. However, aberrations with other symmetries, for example threefold or fivefold, are also frequently observed in projection objectives.
Another major cause for rotationally asymmetric aberrations are certain asymmetric illumination settings in which the pupil plane of the illumination system is illuminated in a rotationally asymmetric manner. Important examples for such settings are dipole settings in which only two poles are illuminated in the pupil plane. With such a dipole setting, also the pupil planes in the projection objective contain two strongly illuminated regions. Consequently, lenses or mirrors arranged in or in the vicinity of such an objective pupil plane are exposed to a rotationally asymmetric intensity distribution which gives rise to rotationally asymmetric aberrations. Also quadrupole settings sometimes produce rotationally asymmetric aberrations, although to a lesser extent than dipole settings.
In order to correct rotationally asymmetric aberrations, U.S. Pat. No. 6,338,823 B1 proposes a lens which can be selectively deformed with the aid of a plurality of actuators distributed along the circumference of the lens. The deformation of the lens is determined such that heat induced aberrations are at least partially corrected.
U.S. Pat. No. 7,830,611 B2 discloses a similar wavefront correction device. In this device one surface of a deformable plate contacts an index matched liquid. If the plate is deformed, the deformation of the surface adjacent the liquid has virtually no optical effect. Thus this device makes it possible to obtain correcting contributions from the deformation not of two, but of only one optical surface. A partial compensation of the correction effect, as it is observed if two surfaces are deformed simultaneously, is thus prevented.
Another way of deforming an optical element, and in particular a thin membrane, is disclosed in U.S. Pat. No. 6,583,850 B2. In one embodiment a cavity is confined by two elliptical membranes. If the gas pressure inside the cavity is changed, the membranes deform in a rotationally asymmetric manner so that a variable astigmatic optical effect is produced.
However, the deformation of optical elements with the help of actuators has various drawbacks. If the actuators are arranged at the circumference of a plate or a lens, it is possible to produce only a restricted variety of deformations with the help of the actuators. This is due to the fact that both the number and also the arrangement of the actuators are fixed. In particular it is usually difficult or even impossible to produce deformations which may be described by higher order Zernike polynomials, such as Z10, Z36, Z40 or Z64.
US 2010/0201958 A1 and US 2009/0257032 A1 disclose a wavefront correction device that comprises a refractive optical element formed as a plate. In contrast to the device described in the aforementioned U.S. Pat. No. 7,830,611 B2, a wavefront correction is not produced by deforming the plate, but by changing its refractive index locally. To this end the plate is provided with thin heating wires that extend over one of its surfaces. With the help of the heating wires a temperature distribution inside the plate can be produced that results, via the dependency dn/dT of the refractive index n on the temperature T, in the desired refractive index distribution. Although even higher order wavefront deformations can be corrected very well with this known wavefront correction device, it is necessary to cool the plate simultaneously, for example by guiding a gas flow over one of its surfaces. However, such a gas flow may itself, as a result of small temperature variations, produce schlieren that compromise the desired effect on the optical wavefront.
U.S. Pat. No. 6,781,668 B2 discloses a lens of a projection objective towards which a number of cooling gas flows are directed. The orientation of the nozzles from which the gas flows emerge can be changed so that the cooling effect on the lenses can be varied. However, also in this known device a certain amount of schlieren is evitable due to the temperature variations in the atmosphere above the lens surface.
Similar wavefront correction devices using gas flows to cool or heat certain areas of a lens are known from U.S. Pat. Nos. 5,995,263, 5,883,704 and 7,817,249 B2. WO 2011/116792 A1 discloses a wavefront correction device in which a plurality of fluid flows emerging from outlet apertures enter a space through which projection light propagates during operation of the projection exposure apparatus. A temperature controller sets the temperature of the fluid flows individually for each fluid flow. The temperature distribution is determined such that optical path length differences caused by the temperature distribution correct wavefront deformations.
A similar concept is described in the aforementioned U.S. Pat. No. 6,583,850 B2. If the gas pressure in a cavity formed between two rigid optical elements is changed, the index of refraction of the gas changes, too. This can be used to modify the refraction at the optical interfaces that confine the cavity. However, this device has only one degree of freedom and is therefore not capable of correcting higher order wavefront deformations.
From the unpublished international patent application PCT/EP2011/004859 (Zellner et al) a wavefront correction device is known in which a plurality of heating light beams are directed towards a circumferential rim surface of a refractive optical element. After entering the refractive optical element, the heating light beams are partially absorbed inside the element. In this manner almost any arbitrary temperature distribution can be produced inside the refractive optical element, but without a need to arrange heating wires in the projection light path that absorb, reflect, diffract and/or scatter projection light to an albeit small, but not negligible extent.